Tool for finding similar sites

One website I have been using a bit lately is Similar Sites. It's useful because when you search for a well known site, you'll get a list of others like it. Then you can do the same search for one of those individually. Of course there is some overlap in the next list that comes up, but you'll get some new sites in it as well. You can keep going for a while like this and turn up some great websites that you probably wouldn't have found via search engines.